package yun2.shuxue;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class FindOne {

	public static void main(String[] args) throws IOException {
		
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String line = br.readLine();
		
		String[] a = line.split(" ");
		
		int ans = 0;
		for(String x : a) {
			ans = ans ^ Integer.valueOf(x);
		}
		System.out.println(ans);
	}

}
